In conventional radio frequency (RF) transmitters, RF power amplifiers are operated linearly in class A, class AB, or class C bias conditions. The problem with these bias classes is that the efficiency of the power stage is limited due to high dissipation in the power stage resulting from either quiescent bias or basic ohmic losses. A large body of research has been devoted to the development of new RF transmitters capable of generating linear RF power using non-linear power amplifiers operating in switch mode in order to increase the power-added efficiency.
Various switch mode techniques have been proposed to improve the power-added efficiency of the power amplifier (PA) stage. These techniques include: i) polar modulation; ii) envelope elimination and restoration; iii) LINC; and iv) delta sigma modulation. In each case, some form of direct modulation is applied in the power amplifier stage. The general concept is to operate the final amplifier stage in switch mode, wherein the final power transistor stage is driven between pinch-off and saturation at the carrier frequency rate or some multiple thereof. By minimizing the percentage of a cycle in which the transistor is in the linear operating region, the power dissipated in the transistor is minimized and a high level of power added efficiency is obtained.
The obvious drawback of this approach is that, to maintain high modulation accuracy and to avoid generating adjacent channel interference, a high degree of envelope linearity is required in the amplifier stage. As a result, high efficiency power amplification techniques focus on maintaining the envelope integrity of the amplified waveform, although the carrier itself will be clipped by switch mode operation. The result is that the in-band modulation accuracy and adjacent channel integrity are preserved, even though a high level of harmonics are generated. The harmonic content is easily filtered after the amplifier stage.
In the polar modulation (or EER) method, the quadrature baseband information is converted to polar components of amplitude and phase. An oscillator operating at the carrier frequency drives the gate of a switch-mode power amplifier (PA) with a constant amplitude, constant frequency signal. The phase component of the complex waveform is used to phase modulate the oscillator controlling the phase of the PA output. The amplitude component is used to amplitude modulate the switch-mode PA by controlling the PA drain bias. Pre-distortion is commonly used to maintain a linear envelope. A high level of carrier harmonics are generated by the switching waveform, so a greater amount of harmonic filtering must be used than is typically required after a linear PA.
One weakness of the polar modulation approach is that the delays through the amplitude modulation path and the phase modulation path are inherently different due to fundamentally different circuit topologies and are subject to differing amounts of variation due to temperature or component variations. A small timing error (>2 nanoseconds) may result in intolerable modulation errors for wideband modulation formats.
In the LINC method, two common frequency components generated from switch mode power amplifier stages are phase modulated then combined so they add and subtract linearly to produce a quadrature modulated carrier. This requires some unique power combining techniques that enable combination of non-coherent waveforms into a load without excessive power dissipation in the combiner. Achieving high efficiency combining has been a significant challenge to implementation of this technique.
The delta-sigma modulation method typically uses a high-order (fourth) delta sigma bandpass loop as a single bit analog to digital converter. This produces a noise transfer function having a notch at the Fs/2 or Fs/4 frequency. A good signal-to-noise ratio may therefore be achieved when a carrier is operated within the notch of the noise transfer function. The single bit output of this loop is used to drive the gate of a switch mode PA. A narrow bandpass filter is used after the power amplifier (PA) to eliminate the broadband noise outside the carrier bandwidth.
One of the challenges of implementing this approach is that the delta-sigma loop must be clocked at twice to four times the carrier frequency. For carriers in the US PCS or IMT-2000 bands, this can be a significant challenge, given current device speeds. Another limitation is that the instantaneous bandwidth of the modulator is limited to about 20 MHz, depending on the order and clock rate of the delta-sigma loop. In addition, the fixed bandpass filter following the PA stage greatly limits the operating band of the PA.
Therefore, there is a need in the art for an improved power amplifier that performs switch mode power amplification without encountering the problems described above.